Switchable dielectric permittivity with temperature and Dc-bias in a semifluorinated azobenzene derivative
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The thermodynamic, optical, structural, and dynamic properties of the semifluorinated (E)-1-(4-octylphenyl)-2-(4-(perfluorooctyl)phenyl)diazene (4) and the corresponding (E)-1,2-bis(4-octylphenyl)diazene (5) have been studied with differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dielectric spectroscopy. 4 combines the azobenzene properties with the fluorophobic effect and gives rise to a responsive material with a temperature and dc-bias-driven switchable dielectric permittivity within the narrower nematic phase. This is caused by the nematic potential that inevitably brings some fluorocarbon chains in proximity to the hydrocarbon chains from adjacent molecules. Frustration is alleviated by reducing the nematic-to-isotropic transition temperature and by increasing the crystalline-to-nematic transition temperature, thus limiting the stability of the nematic phase. Unlike the normal isotropic phase of compound 5, the isotropic phase of compound 4 contains dipoles with short-range orientation correlations. Optimizing the type of interactions may result in materials with applications as molecular switches and electrooptic devices.
KeywordsDielectric permittivity Liquid crystals Molecular dynamics Dielectric Spectroscopy
G.F. acknowledges fruitful discussions with K. Kremer (MPI-P). The current work was supported by the Research unit on Dynamics and Thermodynamics of the UoI, co-financed by the European Union and the Greek state under NSRF 2007-2013 (Region of Epirus, call 18). This work was co-financed by the E.U.—European Social Fund and the Greek Ministry of Development—GSRT in the framework of the program THALIS. Financial support by DFG Priority Program SPP1355, SFP 1459, and the SFB635 is acknowledged.
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